The recent
introduction of many new models onto the market, all of which claim to be
effective at removing unwanted contaminants, has led to confusion and questions,
especially since there is a wide range in cost.
Criteria
There are currently no independent
standards by which all filters are tested. Although most major manufacturers of
treatment systems are currently working together to develope such standards,
their implementation seems years away. Even where goodwill and co-operation
exists, there is plenty of room for legitimate debate over such a complex topic.
How applicable is the EPA Protocol to portable treatment units? Klebsiella
terrigena, the test organism mandated by the Protocol is 0.5+ microns in size,
while some disease-causing bacteria, such as Campylobacter (0.3+ microns) are
much smaller. What concentrations of pathogens should units be expected to deal
with? A unit that reliably copes with lightly contaminated backcountry streams
may be overwhelmed by the raw sewage present in more industrialised rivers.
Should filters, which are not intended to deal with viruses, be denied
certification.
Some of the criteria by which you should judge treatment
systems include-
- Effective removal of
bacteria and protozoa
- Solid,
well-constructed design that does not allow for accidental spillage or leakage
of untreated water or trapped micro-organisms from the filter into the treated
water outlet
- Easily cleaned and
replaceable components and clear guidelines for determining when these
components should be cleaned or replaced
- Cost-effective
long-term treatment of water.
Pore Size
Ratings and Filter Effectiveness
A water filter's pore size rating alone can't be used to judge the
effectiveness of a filter or compare different filters since there is no
regulated, standardized means of determining a filter's pore size rating. To
complicate matters even further, even if the absolute pore size ratings of
different filters were determined by the same means, they can't be used to
directly compare the filtering effectiveness of different filters because carbon
depth filters have pore ratings that may be valid for only a narrow range of
conditions, including water characteristics and operating pressure. The
irregular shape and flexible composition of micro-organisms means that in
certain conditions they can sneak through filters smaller than their usual
diameter. This does not mean that a pore size rating should be disregarded,
however. It is one of the essential measurements by which we are able to
determine a filter's effectiveness, but it should not be used as the only means.
Rather, a manufacturer's specified pore size rating should be verifiable and
considered in conjunction with filter design.
Depth filter materials
Many filtration systems, both for the home and in the field, use
carbon. This depth-type filter is designed to trap contaminants, including silt
and odours, on or close to the surface of the carbon. This is possible due to
carbon's ability to bond with chemical molecules, particles, and organisms, and
the very pattern of the carbon material itself, which is a matrix of dead-ends,
holes, and narrow openings. A serious limitation of removal by adhesion is that
once all the available surface has contaminants bonded to it, water flows by
uncleaned. It's somewhat like a sticky tape in a lint roller - at a certain
point the tape surface is covered and will not pick up any more fluff. While
you can see when a lint brush is no longer working, you cannot tell by looking
when a carbon filter is saturated. While carbon is economical and has the
potential to remove all bacteria, protozoa, and debris, its effectiveness is
questionable, especially when filtering water with strong concentrations of
contaminants. Other points of concern are carbon's tendency to clog and to
allow bacterial colonisation.
Another material commonly used in filters is ceramic. These cleanable
filters remove micro-organisms, silt, and debris from the water, but do not
remove chemicals or odour and taste chemicals. Contaminants larger than the
microporous openings of the filters are trapped on or close to the surface of
the filter, where they can be removed by cleaning. Depending on the quality of
workmanship and material, ceramic filters can be 100% effective in removing
bacteria, larger pathogens, and many viruses. Bacterial colonisation is not as
much of a problem as with carbon filters. Ceramic's biggest disadvantage is its
brittleness. If dropped while satuated with water it can develope irreparable
cracks.
Glass fibre is also used bt some manufacturers. While less abrasion
resistant than ceramic - meaning it will withstand fewer scourings and hence
process less water - its less brittle nature makes it more resistant to being
dropped.
Some depth filters incorporate two or more materials - a first stage
ceramic or glass fibre, and later stage of carbon intended to remove odours and
taste.
Methods of Treatment
Water Filters